1. Technical Field
The present invention generally relates to rubber compositions. The rubber compositions are particularly useful for tire treads, sidewalls and other tire components in a vehicle, e.g., bicycles, motor bikes, passenger automobiles and trucks.
2. Description of the Related Art
It is well known that ozone causes surface cracking of conventional highly unsaturated rubber vulcanizates used in tires when the rubber is placed under strain in an ozone environment. The most severe deterioration occurs when a small number of cracks are formed which grow rapidly into deep, disruptive fissures. These ozone cracks shorten the serviceable life of the tire, especially in the area of the sidewall.
Rubber tires conventionally also have visually exposed external components of one or more rubber compositions which, when exposed to weathering atmospheric conditions and dynamic use, tend to discolor to various degrees and provide an external tire appearance which is not typically uniform in nature. Such tire components having visually exposed surfaces are, for example, tire sidewalls, tire chafers, and tire treads. For example, a black colored tire sidewall may become brown in color as a result of migration of antidegradants to the surface of the rubber composition, together with exposure to atmospheric ozone, ultraviolet light and weathering in general. Such phenomena are well known to those having skill in such art.
Conventional chemical antidegradants have been developed to retard the formation of the ozone cracks occurring under static and dynamic conditions. One class of antidegradants is phenylenediamine compounds. See, e.g., U.S. Pat. Nos. 5,420,354; 5,574,187; 5,840,982 and 6,201,049. Examples of such antidegradants in common use include N-phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine; N-phenyl-N′-isopropyl-p-phenylenediamine; N-phenyl-N′-(1,4-dimethylpentyl)-p-phenylenediamine; N-phenyl-N′-(1-methylheptyl)-p-phenylenediamine; N-phenyl-N′-cyclohexyl-p-phenylenediamine; mixed diaryl-p-phenylenediamines; N,N′-diphenyl-p-phenylenediamine; N,N′-di-beta-naphthyl-p-phenylenediamine; N,N′-bis(1,4-dimethylpentyl)-p-phenylenediamine; N,N′-bis(1-ethyl-3-methylpentyl)-p-phenylenediamine; N,N′-bis(1-methylheptyl)-p-phenylenediamine; N-phenyl-N′-p-toluenesulfonyl-p-phenylenediamine and blends of these materials.
While the use of these well known paraphenylenediamine compounds has improved ozone protection under both static and dynamic conditions, these compounds have a very strong tendency to stain, discolor, and to form a noticeable brownish bloom on the exposed surfaces. In tires, which are the largest application in which ozone protection is required, an objectionable, noticeably brown, dull surface is typically created on the sidewall by the paraphenylenediamine compounds.
In addition, the retreading of tires is a common practice in the tire industry, as it allows the original tire carcass or casing, to be reused once the tread has served its useful life. Typically, the retreading of tires, and particularly truck tires, is carried out not only once, but a plurality of times in some cases. This is desirable in that it is less costly to replace the tread instead of purchasing a new tire. Also, it reduces the environmental impact of tire disposal. In order to be able to retread a tire multiple times, the structural integrity of the carcass must be maintained for as long as possible. However, the structural integrity of a tire carcass can degrade over time due to, for example, oxidation and/or flex fatigue, which could limit the number of times the tire is retreaded. This is particularly a problem for truck tires.
Accordingly, it would be desirable to provide rubber compositions having improved ozone resistance while also being able to mask, retard, and/or substantially eliminate for an extended period of time, such dulling and/or discoloring (usually browning) of the exposed surface of a tire component such as a sidewall due to such migration of various antidegradants and the like and/or exposure to atmospheric conditions. It would also be desirable to provide a tire carcass having improved structural integrity for a prolonged period of time so as to maximize the number of times a tire could be retreaded.